First off... the engine will be placed in an airboat, I've come to this forum because nitro airplane engines are what's used in nitro airboats. People that know a lot about nitro plane engines are gonna be here.

Ok... I want to put a .75 in a Big Dumas Swamp Buggy and need help choosing which one. Price is a concern and so is weight(top heavy) so price puts O.S. out of the picture (double what others cost), and no to Super Tigre for weighing noticeably more than the two I'm considering, a Tower Hobbies .75 or a GMS .76. Which one is the better engine? They are only $5 apart. And if GMS, it's offered in both front needle valve and remote needle valve, I don't know the difference between the two, which would the better choice be? BTW tower's engine is front needle valve.

Third question... what size 3 blade prop(puller) would I use on this .75? not a 2 blade because it wouldn't fit and CG wouldn't be great. So if there is more than one appropriate blade length 3 blade prop for a .75, I think I would want one on the smaller side.

Thanks for the help.

it should say in the manual

I am assuming that you are going to use this in a pusher configuration like REAL swamp buggies so you will need a pusher prop. I'd also pick a remote needle valve to keep your hand away from the prop. Prop size?? I'm guessing (and it's just a guess) 3 blade 12x6 or in that range.

Most likely none of this is in the manual.

Look at the weight with mufflers. The GMS is only an ounce less than the Super Tigre .75 without muffler, about the same oz difference with. The tower engine is 4 to 5 oz less with muffler than the other two.

Dont bother with the ST.it takes some skill to tune them and possibly some carb mods to keep them running reliable.
From what I have heard,and I do not have first hand experience,but many posts pointed that the tower 75 is a real reliable screamer.Used in a few airplanes on this board and with parts available,I would probably go for that one.Not sure on noise restrictions,I heard the tower motors were loud.

Bruce, actually, the mounts are set up for it to be a puller, a major majority of rc airboats are pullers, mainly because of the advantage of the props air cooling the engine. And thanks for informing me about remote needles, now I see what the difference is, just a re-located needle, seems safer so I'll go with it, good suggestion.

The Tower Hobbies engine is a remote needle, I said wrong in my first post, so I'd go remote needle with either engine I choose. So plane guys, GMS or Tower Hobbies engine? GMS is a good brand, rite?

Thanks.

edit: The picture(on websites selling the kit) simply shows the Big Swamp Buggy as a puller like most of the rc airboats are. The reason for the two blade prop in the picture is they recommend the underpowered engine(.40) making for 15mph(ish) speed so that's why it's a 2 blade prop in the picture.

Never owned either. But have (and still like) Super Tigres. ;-)

Dont seem to recall many GMS engines in aircraft,but could be wrong.

Charlie P, yeah I know Super Tigre makes engines are good but they weigh almost twice what the other two I'm looking at do. So due to CG / flipping the boat over Super Tigre just isn't my best option on this one...

That's interesting... they have a full line of airplane engines just like the other companies so I figured they'd be fairly common since their prices seem to be the cheapest. http://www3.towerhobbies.com/cgi-bin...S+++&search=Go

Edit: I just discovered GMS has 2.5 hp and 15,000 rpm, Tower is 2.2 hp, 15,600 rpm, maybe that could help someone decide which engine I would benefit from more?
Also I need to get some of my facts straight. The 20 oz. GMS engine, that's without muffler, other weights I gave were with muffler. Also the super tiger may weigh a fair amount more but I don't know what I was thinking when I said double, not even close...
Thanks.

Look again. A ST .75 is 20 oz without muffler & 27 oz with. I make that 12% heavier than the .76 GWS and only 4% heavier than the Tower .75

That's a far piece from 100% heavier (2X)

I can't think of any of the 40 members in my club that fly a GWS or Tower engine. O.S., SuperTigre, Thunder Tiger in the glow two strokes. Rarely others.

Check out Kangke/SuperKraft if you want cheap. I'm still tryng to get mine to run right after two years of trying. Cheap engines can be a misery and we do this stuff for fun.

This plane, and boat, guy says go with the Tower.

For props, the manual suggests breaking it in with a 12x6 prop and says a 13x6 could also be used. Many of the 3 blade recommendations I've seen suggest stepping down 1 inch. This would mean an 11x6 or 12x6 3 blade prop should work. Keep in mind that another way to reduce the diameter is to go up in pitch. Going down 1 inch in diameter, but up one inch in pitch gives approximately the same load on the engine. So, a 2 blade 11x7 or 11x8 might give similar results. The increased pitch will possibly give you higher speeds. The reason I say possibly is that by moving less air, but doing it at a higher speed, they may not be able to overcome the drag from the water as well as a larger diameter, but smaller pitch. Experimentation will be needed to find the best prop. As you're doing this experimentation, it would be a good idea to buy, or borrow, a tach so you can measure the RPM range of each prop. This will help you ensure that you're not over or under propping the engine.

You seem very concerned about making it to top heavy. That being the case have you compared the weight of the average recomended .40 size engine to the .75 you are now concidering? In an airframe that is designed for a .40 size engine we can usually get away with going for a .50-.55 size engine without to much difficulty. A .75 would put a plane way out of ballance and require motor mount modifications. I understand price is a concern but I would think reliability would be as well. I would not put anything on the water that I could not trust to stay running and in this hobby... you get what you pay for. OS .55 AX would be my choice. I have one in a Funtana 40s and it has never dead sticked in on me.

I've been doing some prop tests on a newly purchased .75 and found a couple of good performers.

The new line of Evolution reinforced nylon props looked good and I found the 14x6 worked good. A Zinger 13x7 did as well. Both were 2bladers and you asked specifically about 3bladers. I tried two so far and one gave less performance than the two good 2bladers. Basically, the reason for the poor performance with the one was because it wasn't enough load on the engine. The rpm was the highest the engine has shown with any prop I've tested on it so far. The poor performer was a 12x6(3) that turned over 12,500rpm on the ground.

The best prop so far on the .75 on that airplane has been a Master Airscrew 12x8(3). The engine turns the least rpm on the ground of any tested so far, but it pulls like a bull in the air. It's what I've flown on the plane since trying it. And I've not been much interested in more prop tests since testing it. The engine turned it just over 9,100 rpm on a 90+degree, low pressure day. I haven't bothered to tach it since the test day.